In a typical drilling operation, a drill bit is rotated while being advanced into a rock formation. There are several types of drill bits, including roller cone bits, hammer bits and drag bits. There are many drag bit configurations of bit bodies, blades and cutters.
Drag bits typically include a body with a plurality of blades extending from the body. The bit can be made of steel alloy, a tungsten matrix or other material. Drag bits typically have no moving parts and are cast or milled as a single-piece body with cutting elements brazed into the blades of the body. Each blade supports a plurality of discrete cutters that contact, shear and/or crush the rock formation in the borehole as the bit rotates to advance the borehole. Cutters on the shoulder of drag bits effectively enlarge the borehole initiated by cutters on the nose and in the cone, or center, of the drill bit.
FIG. 1 is a schematic representation of a drilling operation 2. In conventional drilling operations a drill bit 10 is mounted on the end of a drill string 6 comprising drill pipe and drill collars. The drill string may be several miles long and the bit is rotated in the borehole 4 either by a motor proximate to the bit or by rotating the drill string or both simultaneously. A pump 8 circulates drilling fluid through the drill pipe and out of the drill bit flushing rock cuttings from the bit and transporting them back up the borehole. The drill string comprises sections of pipe that are threaded together at their ends to create a pipe of sufficient length to reach the bottom of the borehole 4.
Cutters mounted on blades of the drag bit can be made from any durable material, but are conventionally formed from a tungsten carbide backing piece, or substrate, with a front facing table comprised of a diamond material. The tungsten carbide substrates are formed of cemented tungsten carbide comprised of tungsten carbide particles dispersed in a cobalt binder matrix. The diamond table, which engages the rock formation, typically comprises polycrystalline diamond (“PCD”) directly bonded to the tungsten carbide substrate, but could be any hard material. The PCD table provides improved wear resistance, as compared to the softer, tougher tungsten carbide substrate that supports the diamond during drilling.
Cutters shearing the rock in the borehole are typically received in recesses along the leading edges of the blades. The drill string and the bit rotate about a longitudinal axis and the cutters mounted on the blades sweep a radial path in the borehole, failing rock. The failed material passes into channels between the bit blades and is flushed to the surface by drilling fluid pumped down the drill string.
Some materials the bit passes through tend to clog the channels and reduce the efficiency of the bit in advancing the borehole. As the bit fails materials such as shale at the borewall, the material quickly absorbs fluid and can form clays that are sticky. Clays can form ribbons as it is cut from the bore that agglomerate and can cling to the surface of the bit in the channels. This narrows the channels and can inhibit flushing of new material to the surface. The material expands as it absorbs water and pressure increases in the channels of the bit. While this pressure in the channel can help flush less sticky material from the channel, the pressure can cause clay to stick to the channel walls. This causes the bit to bog down and limits the volume of new material that can be processed through the channel.
Bits configured to advance boreholes through materials of finer consistency that form clays and flush the failed materials more efficiently out of channels without clogging can be advantageous.